1. Field of the Invention
The present invention is directed to the field of acoustical testing and more specifically to the area of measuring the transmission loss characteristics for various sheet materials.
2. Description of the Prior Art
Several attempts have been made to provide accurate, reproducible measurements of acoustical transmission losses in structural materials. These measurements are important to those involved in designing such items as buildings, machinery mountings, aircraft, as well as automotive vehicles, since noise isolation is an important factor for insuring longer usable life of the items and human operator and passenger comfort.
An article entitled "Damping Treatments for Noise and Vibration Control" by David I. G. Jones, appeared in Sound and Vibration in July 1972 includes a survey of materials and formation techniques that can be utilized to control noise transmission and vibration induced failures in structures. A representative plot of Transmission Loss (TL) vs. Frequency, is shown in FIG. 1, of the article, for a typical structure having transmission loss characteristics classified into three broad regions. In a low range of frequencies, the transmission losses are considered to be mainly "stiffness controlled" while in the intermediate range of frequencies, transmission losses are considered to be "damping controlled" and at higher frequencies, transmission losses are essentially considered to be "mass controlled". It is noted that the three regions are distinct and recognizable in most materials, although the influence of damping extends into the adjacent regions, as well. It is also noted that increasing the stiffness of the material will cause the transmission losses to increase in that material in the stiffness controlled region of frequencies. Similarly, increasing the damping of the material will decrease the resonant vibration amplitude and increase the transmission loss. Increasing the mass of the structure will, as expected, increase the transmission loss in the mass control region but may not have an effect on the vibration amplitude.
It is recognized that the most desirable material, in those installations where a high transmission loss is desired over all frequencies, would be a material which is stiff, heavy and highly damped. On the other hand, in installations such as vehicles and aircraft, weight considerations are quite important and therefore the use of heavy materials is not practical. The Jones article acknowledges that consideration and describes several methods of forming and bracing materials so as to change the response spectra. The Jones article also discusses the use of damping materials and their effects on the response spectra of the structures.
Another article entitled "Method for Determining Sound Transmission Loss in the Field", by Albert London, appeared in the Journal for Research of the National Bureau of Standards, vol. 6, May 1941. The London article describes a method of determining the transmission loss of a wall or floor partition by measuring the difference in sound levels existing in two adjacent rooms which have the partition as a separating wall or floor. In one room, a loud speaker unit havig six differently oriented loudspeakers is rotated to give a particular sound pattern that is said to be uniformly distributed throughout the room. Two displaced microphones are positioned an equal distance from the panel surface. On the other side of the panel in the "quiet" room, a similar pair of displaced microphones is positioned. The distance measurement from the surface to the microphone pairs for both the "quiet" and "noisy" rooms are discussed for each of the several measurements made for each of the individual vibration frequencies tested. An alternate method is also described which utilizes a pressure-gradient microphone in place of each pair of conventional microphones.
In terms of automotive acoustical control, the general method employed, to decrease noise levels in the vehicle, is to drive the vehicle on a test track having a known source of road noise and measuring the noise level within the vehicle by use of a sound pressure level meter. When noise levels are considered too high, the transmission point for that noise is attempted to be located and appropriate stiffening or application of carpeting or other composite materials are employed to inhibit the transmission of the noise until a desirable level is reached. Such methods are purely empirical since they are based upon the experience of the test personnel and fail to provide an acoustical standard by which the various materials may be specified to meet.